Waveguide type radar apparatus having polarization converter



M. H. EASY 2,952,017

TYPE RADAR AZ-TPARATUS HAVING POLARIZATION CONVERTER Sept. 6, 1960 WAVEGUIDE Filed June 17, 1957 Patented Sept. 6, 1960 WAVEGUIDE TYPE RADAR APPARATUS HAV- ING POLARIZATION CONVERTER Maurice Henry Easy, London, England, assignor to The Decca Record Company Limited, London, England, a British company Filed June 17, 1957, Ser. No. 666,193

11 Claims. (Cl. 343-756) This invention relates to radar apparatus and more particularly to radar apparatus of the kind in which the direction of polarisation of the radiated signals may be adjusted.

According to this invention in radar apparatus having a transmitting antenna comprising a reflector or lens together with a waveguide-fed horn for directing the output of the transmitter on to the reflector or lens, there are provided a rotatable waveguide unit, means including a rotatable coupling for feeding the output from the radar transmitter into said rotatable waveguide unit, the rotata le coupling being arranged to provide a linearly polarised signal in said waveguide unit having a plane of polarisation which is fixed with respect to said unit and may thereby be rotated by rotation of the unit, a square waveguide coupled to said rotatable waveguide unit with the waveguide walls at 45 to the plane of polarisation of the signal in said rotatable waveguide unit, a circular polariser comprising an adjustable phase shifter in said square waveguide for shifting the phase of the component polarised in a plane parallel to one of the walls of the square waveguide and coupling means for feeding the output of said square waveguide to said horn. Most convenently a single antenna system is used for both transmitting and receiving and, in that case, the adjustable means for rotating the plane of polarisation and the circular polariser are preferably arranged so as to be effective on both the transmitted and received signals. It would however, be possible to have separate transmitting and receiving antenna systems, in which case the receiving systems would be provided with means for altering the polarisation of the received signals.

Most transmissions on microwave frequencies are either horizontally or vertically polarised. If, for example, interference is being experienced from a horizontally polarised transmission, by using the arrangement of the present invention this interference can be considerably reduced or possibly eliminated by adjusting the radar apparatus to operate with vertically polarised transmission and reception and vice-versa. Linear polarisation, at any other angle, being transmitted from the interfering station, may be eliminated or greatly reduced by adjusting the ap paratus to operate with polarisation at right angles to that of the interfering signals from other stations transmitting on a frequency within the pass-band of the receiver of the station concerned. Furthermore with the apparatus of the present invention it is possible to transmit and receive signals with circular polarisation which is either righthanded or left-handed or with elliptical polarisation and it is possible to vary the ratio of the major to minor axis in the case of elliptical polarisation and to rotate the major axis of the ellipse. By making such further provision, it is possible to eliminate or reduce unwanted signals whatever form of linear, circular or elliptical polarisation may be transmitted from the interfering station.

The aforementioned rotatable coupling for feeding the output from the radar transmitter to said rotatable waveguide unit may be arranged to feed the output from the transmitter as a linearly polarised transverse electric wave into a waveguide forming part of said waveguide unit and having a portion parallel to and co-linear with the rotatable coupling axis so that the plane of polarisation in said waveguide portion may be rotated by rotation of said waveguide portion.

Such a rotatable coupling may be formed, for example, by two relatively rotatable coaxial lines joined end to end and rotatable about their common axis through which the signals are fed as TEM waves, the signals being fed from the coaxial line into a rectangular guide (as transverse electric waves) having its axis at right angles to the axis of rotation, this rectangular guide having bends to provide the aforementioned portion co-linear with the coupling axis. v

in order to avoid any necessity for rotating the horn, it is preferable to construct the aforementioned coupling means for feeding the output of said square waveguide to said horn as a rotatable coupling arranged not to affect the plane of polarisation. These coupling meansmay comprise a rotatable coupling formed by two coaxial relatively rotatable circular waveguides coaxial with the square waveguide and horn.

The aforementioned adjustable phase shifter conveniently comprises a sheet of dielectric material arranged in the guide to lie with its plane parallel to one of the sides of the guide, the sheet being adjustable in a direction transversely to its plane. To produce a circularly polarised radiated signal, the phase shift would then be adjusted to give the relative phase shift of 90 between the two components. Both right and left-handed circular polarisation can be obtained if the phase shifter is adjustable to produce relative phase shifts of plus or minus or the equivalent. If the phase shift is 0 or then a linearly polarised output would be obtained and, by adjusting the phase shift to obtain phase shifts between these values, an elliptically polarised output can be obtained. With this construction, a linearly polarised signal is always fed into the elliptical polarising unit. If the phase shifter in this unit is set to give a linearly polarised output, then the plane of polarisation can be rotated by turning the rotatable part of the assembly. If the phase shifter is set to give an elliptically polarised output, then the major axis of the ellipse may be rotated by turning the. rotatable part of the assembly. It will be noted that, if the same feed system is used both for the transmitted and the received signals, whatever the angular position of the rotatable part of the assembly or the setting of the phase shifter, a linearly polarised input will be provided for the receiver.

In an elliptical polarising unit as described above having a square waveguide with the planes of its walls at 45 to the plane of polarisation of the input signalsfrom the transmitter, there may be provided between the rotatable coupling and the square waveguide, a length of rectangular or square guide through which the signals from the transmitter pass with the plane of polarisation parallel to the walls and, in this length of guide, there may be provided, between the phase shifter and the rotatable coupling for feeding the output of the transmitter into the rotatable waveguide unit, means for attentuating any signal components polarised transversely to the required linearly polarised signals. These attentuating means may comprise, for example, a grating across the waveguide. Such a grating would prevent any received signals of unwanted polarisation passing to the receiver. In an arrangement in which the rotatable waveguide unit, horn and reflector or lens are used both for transmission and reception, the attenuating means may comprise, additionally or alternatively to the grating, an auxiliary guide with a dummy load or loads, said auxiliary guide being coupled to said length of rectangular or. square waveguide by a slot arranged 'to elfec't coupling only for 3 waves having the electric vector in the unwanted plane. If, for any purpose, these normally unwantedsignals should be required, this further guide, instead of having a dummy load, may be connected to a suitable receiver.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which:

'Figure 1 is a diagrammatic representation of part of a pulse radar apparatus; and

Figures 2, 3, 4, 5, 6, 7 and 8 are transverse sections through the waveguide and horn assembly for Figure 1 along the lines 22 to 8-8 respectively.

Referring to Figure 1 there is shown diagrammatically a pulse transmitter 10 for producing short duration pulses of microwave energy which are fed through a duplexer 11 into a rectangular waveguide 12. These pulses are eventually radiated by a horn 13 onto a reflector 14 forming part of an antenna for producing a narrow directional beam of radiation. In the known manner this beam may be scanned by rotation or oscillation of the antenna system about an axis transverse to the direction of the beam. Echoes of these pulses reflected from radar targets strike the reflector 14 and are picked up by the horn 13 and eventually fed back through the waveguide 12 to the duplexer 11 and thence passed to a receiver 15. After detection, the received echoes may be displayed in the known manner. The signals fed into the waveguide 12 from duplexer 11 are transverse electric signals linearly polarised with the electric vector parallel to the shorter sides of the guide as shown by the arrow 20 in Figure 2. The waveguide 12 as shown in Figure 1 is coupled by a rotatable coupling 21 to a further rectangular waveguide 22 having its axis at right angles to the axis of rotation. The rotatable coupling 21 is formed of two relatively rotatable circular co-axial lines 23, 24 with a coupling member 25 extending axially through them, this coupling member being arranged to extend into the waveguides 12 and 22 at right angles to their axes so that the signals pass through the rotatable coupling 21 as TEM waves and are fed into the rec tangular guide 22 as transverse electric waves with the electric vector parallel to the shorter sides of the guide. It will be seen that the electric vector will always remain in this plane despite any relative rotation of the guides 12 and 22 at the rotatable coupling 21. This rectangular guide 22 includes bends so that there is a portion 26 with its axis coincident with the axis of rotation of the coupler 21. The rectangular waveguide 26 is coupled by a transition section 27 to a square waveguide 28 having its walls parallel to the planes of the walls of the guide 26 so that the electric vector of the signals from the transmitter is parallel to one pair of walls of the square guide 28. From the square waveguide 28 the signals are fed through a further transition section 29 into another similar section square waveguide 30 having its walls in planes at 45 to the planes of the walls of the guide 28 and thus at 45 to the plane of polarisation of the signals entering the guide 30 from the transmitter.

Within the square section guide 30 there is provided an adjustable phase shifter comprising a sheet of dielectric material 31 (Figure arranged in the guide to lie with its plane parallel to one of the sides of the guide, the sheet being mounted on dielectric rods 32 so asto be adjustable in position across the guide in a direction transversely to this plane. Spring means (not shown) may be provided biasing the sheet in one direction andv a power-driven cam or the like provided for effecting movement in the other direction so that the position of the phase shifter can be remotely controlled. The polarisation of the incident signals in the guide 30 from the transmitter is indicated by the arrow 33 in Figure 5 and these may be considered as formed of two components in the direction of the arrows 34 which are at 45 to the direction of the arrow 33. The sheet of dielectric fleeting surface.

material 31 will introduce a phase shift in one of the components, but not in the other, the amount of phase shift depending on the position of the sheet 31 across the guide 30. The maximum phase shift will occur when the sheet 31 is central in the guide and the minimum phase shift when it is at one or the other side. This phase shifter thus enables the relative phase of the two components represented by arrows 34 to be varied and hence the output signal, after passing the phase shifter, can be considered as in general an elliptical polarised signal. When the sheet 31 is at one side of the guide 30, however, there will be substantially no phase shift introduced and hence the output will be a linearly polarised signal. The phase shifter is constructed to provide a phase shift of at least and preferably so that the output may be varied from linearly polarised at least to circularly polarised and, if 180 of phase shift is available, on to linear polarisation in a direction at right angles to the former plane of polarisation. From the square section waveguide 30 the signals pass through a further transition section 40 leading to a circular waveguide 41 which is coupled by a rotatable coupling 42 to a further fixed circular waveguide '43. This circular waveguide, by means of a transition section 44, leads to a square section guide 45 which feeds the aforementioned horn 13 which is of a shape formed by flaring out two opposite parallel sides of the square section guide 45.

It will be seen that a linearly polarised signal is always fed into the elliptical polarising unit. 7 If the phase shifter in this unit is set to give a linearly polarised output, then the plane of polarisation can be rotated by turning the rotatable part of the assembly between the rotatable couplings 21 and 42. If the phase shifter is set to give an elliptically polarised output, then the major axis of the ellipse may be rotated by turning the rotatable part of the assembly. It will be noted that if circular polarised or elliptically polarised signals are transmitted, similarly polarised signals with the same direction of rotation received by the horn 13 would be converted by the elliptical polariser and rotatable waveguide unit into linearly polarised signals at the duplexer 11 with the plane of polarisation similar to that of the transmitted signals.

In the square section guide 28 there is provided a grating 50 comprising a number of thin wires arranged transversely to the direction of polarisation of the transmitted signals to act as an attenuator for any unwanted components of signals having a transverse direction of polarisation. The square waveguide 28 also has a rectangular slot 51 (Figure 4) in one of its walls opening into a length of rectangular waveguide 52 which slot is arranged to be non-radiating for the signals of the required polarisation but to couple signals of transverse polarisation strongly into the rectangular waveguide 52 where there is provided a dummy load 53. If for any purpose these normally unwanted signals should be required, they may be fed to a suitable receiver instead of the dummy load 53. When reflected signals are received by the horn 13, their polarisation will depend on the polarisation of the radiated signals and, if they are circularly or elliptically polarised, on the nature of the re- Signals reflected from a surface formed of two planes at an angle so that double reflection takes place would always be returned to the radar apparatus with the same polarisation as the radiated signals, that is to say if they were linearly polarised they would be returned as similarly linearly polarised signals and if they were elliptically polarised with a right-handed direction of rotation they would be returned as similar elliptically polarised signals also with a right-handed direction of rotation. On the other hand, a flat plate or spherical drops such as drops of rain, although returning linearly polarised signals as' similar linearly polarised signals, will return circularly or elliptically polarised signals with an opposite handed direction of rotation. It will be seen that the received signals at the horn 13 if they are polarised similarly to the transmitted signals will be passed through to the waveguide 12 but, if they are circularly polarised and are reflected from a flat plate or from spherical rain drops, they will, after passing through the adjustable polariser 30, not be able to pass through the grating 50 and will pass through the slot 51 into the dummy load 53. Thus the arrangement serves to pass signals reflected from targets having complex reflecting surfaces such as are usually present on for example ships or aircraft, but will reject signals from a rainstorm or interfering signals of some different form of polarisation. It is found in practice that since rain drops are not exactly spherical, to get maximum rejection of response from rain, the radiated signals are preferably elliptically polarised and the adjustable phase shifter 31 enables the ellipticity to be varied so as to secure maximum rejection as is described in the specification of co-pending application No. 620,019.

I claim:

1. In radar apparatus having a transmitter and a transmitting antenna comprising a focussing device and with a waveguide-fed horn for directing the output of the transmitter onto the focussing device; the combination of a rotatable waveguide unit, means including a rotatable coupling for feeding the output from the transmitter into said rotatable waveguide unit, the rotatable coupling being arranged to provide a linearly polarised signal in said waveguide unit having a plane of polarisation which is fixed with respect to said unit and may thereby be rotated by rotation of the unit, a square waveguide coupled to said rotatable waveguide unit with the waveguide walls at 45 to the plane of polarisation of the signal in said rotatable waveguide unit, a circular polariser comprising a transversely movable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the Walls of the square waveguide for shifting the phase of the component polarised in that plane and coupling means for feeding the output of said square waveguide to said born.

2. Radar apparatus as claimed in claim 1 wherein said rotatable coupling for feeding the output from said transmitter to said rotatable waveguide unit is arranged to feed the output from the transmitter as a linearly polarised transverse electric wave into a waveguide forming part of said waveguide unit and having a portion parallel to and co-lin'ear with the rotatable coupling axis so that the plane of polarisation in said waveguide portion may be rotated by rotation of said waveguide portion.

3. Radar apparatus as claimed in claim 1 wherein said coupling means for feeding the output of said square waveguide to said horn comprises a rotatable coupling arranged not to affect the plane of polarisation.

4. Radar apparatus as claimed in claim 1 wherein the coupling means for feeding the output of said square waveguide to said horn comprises a rotatable coupling formed of two co-axial relatively rotatable circular waveguides co-axial with the square waveguide and horn.

5. In radar apparatus having a transmitter and a transmitting antenna comprising a focussing device and a waveguide-fed horn for directing the output of the transmitter onto the focussing device; the combination of a rotatable waveguide unit, a rotatable coupling arranged to couple said transmitter to said rotatable waveguide unit and formed of two relatively rotatable co-axial lines joined end to end and rotatable about their common axis through which lines the signals are fed as TEM waves, the rotatable coupling further including a rectangular guide into which the signals are fed from the co-axial lines as transverse electric waves, the rectangular guide having bends to provide a portion parallel to and colinear with the coupling axis whereby the rotatable coupling provides a linearly polarized signal in said wave- 6 guide unit having a plane of polarization which is fixed with respect to said unit and may thereby be rotated 'by rotation of the unit, a square waveguide coupled to said rotatable waveguide unit with the waveguide walls at 45 to the plane of polarization of the signal in said rotatable waveguide unit, a circular polarizer comprising a transverselymovable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the walls of the square waveguide forming an adjustable phase shifter for shifting the phase of the component polarized in a plane parallel to one of the walls of the square waveguide, and coupling means for feeding the output of said square waveguide to said born.

6. In radar apparatus having a transmitter and a trans mitting antenna comprising a focussing device and a waveguide-fed horn for directing the output of the transmitter onto the focussing device; the combination of a rotatable waveguide unit, means including a rotatable coupling for feeding the output from said transmitter into said rotatable Waveguide unit, the rotatable coupling being arranged to provide a linearly polarized signal in said waveguide unit having a plane of polarization which is fixed with respect to said unit and may thereby be rotated by rotation ofsaid unit, said rotatable waveguide unit including a length of guide having adjacent walls at right angles and arranged to pass the signals from the transmitter with their plane of polarization parallel to one pair of walls of this length of guide, means in said length of guide for attenuating any signal components polarized transversely to said one pair of Walls, a square Waveguide coupled to said rotatable waveguide unit with the Waveguide walls at 45 to the plane of polarization of the signal in said rotatable waveguide unit, a circular polarizer comprising a transversely movable sheet of dielectric material arranged in said square'waveguide with its plane parallel to one of the walls of the square wave guide forming an adjustable phase shifter for shifting the phase of the component polarized in a phase parallel to one of the walls of the square waveguide, and coupling means for feeding the output of said square waveguide to said horn.

7. Radar apparatus as claimed in claim 6 wherein the rotatable waveguide unit, horn and focussing device are used both for transmission and reception and wherein said attenuating means comprise an auxiliary guide with a dummy load, said auxiliary guide being coupled to said length or guide with its adjacent walls a right angles by a slot arranged to efiect coupling only for waves having the electric vector in the unwanted plane.

8. In radar apparatus having a transmitter and a transmitting antenna comprising a focussing device and a waveguide-fed horn for directing the output of the transmitter onto the focussing device; the combination of a rotatable waveguide unit, a rotatable coupling arranged to couple said transmitter to said rotatable waveguide unit and formed of two relatively rotatable co-axial lines joined end to end and rotatable about their common axis through which lines the signals are fed as TEM waves, the rotatable coupling further including a rectangular guide into which the signals are fed from the co-axial lines as transverse electric waves, the rectangular guide having bends to provide a portion parallel to and colinear with the coupling axis whereby the rotatable coupling provides a linearly polarized signal in said Waveguide unit having a plane of polarization which is fixed with respect to said unit and may thereby be rotated by rotation of the unit, a square waveguide coupled to said rotatable waveguide unit, a circular polarizer comprising a transversely movable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the walls of the square waveguide forming an adjustable phase shifter for shifting the phase of the component polarized in a plane parallel to one of the walls of the square waveguide, and coupling means for feeding the output of said square waveguide to said horn.

9. In radar apparatus having a transmitter and a transmitting antenna comprising a focusing device and a waveguide-fed hornfor directing the output of the transmitter onto the focusing device; the combination of a rotatable waveguide unit, a rotatable coupling arranged to couple said transmitter to said rotatable waveguide unit and formed of two relatively rotatable co-axial lines joined end to end and rotatable about their common axis through which lines the signals are fed as TEM waves, the rotatable coupling further including a rectangular guide into which the signals are fed from the co-axial lines as transverse electric waves, the rectangular guide having a portion parallel to and co-linear with the coupling axis whereby the rotatable coupling provides a linearly polarized signal in said waveguide unit having a plane of polarization which isfixed with respect to said unit and may thereby be rotated by rotation of the unit, a square Waveguide coupled to said rotatable waveguide unit with the waveguide walls non-parallel to the plane of polarization of the signal in said rotatable waveguide unit, a circular polarizer comprising a transversely movable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the Walls of the square waveguide forming an adjustable phase shifter for shifting the phase of the component polarized in a plane parallel to one of the walls of the square waveguide, and coupling means for feeding the output of said square waveguide to said born,

10. In radar apparatus having a transmitter and a transmitting antenna comprising a focusing device and a Waveguide-fed horn for directing the output of the transmitter onto the focusing device; the combination of a rotatable waveguide unit, means including a rotatable coupling for feeding the output from said transmitter into said rotatable waveguide unit, the rota-table coupling being arranged to provide a linearly polarized signal in said Waveguide unit having a plane of polarization which is fixed with respect to said unit and may thereby be rotated by rotation of said unit, said rotatable waveguide unit including a length of guide having adjacent walls at right angles and arranged to pass the signals from the transmitter with their plane of polarization parallel to one pair of walls of this length of guide, means in said length of guide for attenuating any signal components polarized transversely to said one pair of walls, a square waveguide coupled to said rotatable waveguide unit with the waveguide walls non-parallel to the plane of polarization of the signal in said rotatable waveguide unit, a circular polarizer comprising a transversely movable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the walls of the square Waveguide forming an adjustable phase shifter for shifting the phase of the component polarized in a phase parallel to one of the walls of the square Waveguide, and coupling means for feeding the output of said square waveguide to said horn.

ll. In radar apparatus having a transmitter and a transmitting antenna comprising :a focusing device and a waveguide-fed horn for directing the output of the transmitter onto the focusing device; the combination of a rota-table waveguide unit, a rotatable coupling arranged to couple said transmitter to said rotatable waveguide unit and formed of two relatively rotatable co-axial lines joined end to end and rotatable about their common axis through which lines the signals are fed as TEM waves, the rotatable coupling further including a rectangular guide into which the signals are fed from the co-axial line as transverse electric waves, therectangular guide having a portion parallel to and colinear with the coupling axis whereby the rotatable coupling provides a linearly polarized signal in said waveguide unit having a plane of polarization which is fixed with respect to said unit and may thereby be rotated by rotation of the unit, a square Waveguide coupled to said rotatable waveguide unit, a circular polarizer comprising a transversely movable sheet of dielectric material arranged in said square waveguide with its plane parallel to one of the walls of the square waveguide forming an adjustable phase shifter for shifting the phase of the component polarized in a plane parallel to one of the walls of the square waveguide, and coupling means for feeding the output of said waveguide to said horn.

References Cited in the file of this patent UNITED STATES PATENTS 

